Molecular sieves are a commercially important class of crystalline materials. They have distinct crystal structures with ordered pore structures which are demonstrated by distinct X-ray diffraction patterns. The crystal structure defines cavities and pores which are characteristic of the different species.
Molecular sieves identified by the International Zeolite Associate (IZA) as having the structure code CHA are known. These materials are characterized as having three-dimensional 8-membered ring (8MR) pore/channel systems containing double-six-rings (D6R) and cages. CHA-type molecular sieves, especially high-silica CHA-type molecular sieves having a SiO2/Al2O3 molar ratio (SAR) of at least 10, are employed in a variety of commercial applications, such as in methanol-to-olefin catalysis and in selective catalytic reduction of nitrogen oxides (NOx).
U.S. Pat. No. 4,544,538 discloses a CHA-type molecular sieve designated SSZ-13 which is prepared in the presence of a N-alkyl-3-quinuclidinol cation, a N,N,N-trialkyl-1-adamantammonium cation and/or a N,N,N-trialkyl-2-exoaminonorbornane cation as the structure directing agent (SDA). According to this patent, SSZ-13 typically has a SiO2/Al2O3 molar ratio of 8 to 50.
An all-silica crystalline molecular sieve having the CHA framework type has been hydrothermally synthesized using a N,N,N-trimethyl-1-adamantammonium cation as the structure directing agent, but the synthesis requires the presence of concentrated hydrofluoric acid. See M-J. Diaz-Cabañas et al., “Synthesis and Structure of Pure SiO2 Chabazite: the SiO2 Polymorph with the Lowest Framework Density,” Chem. Commun. 1998, 1881-1882.
U.S. Pat. No. 6,709,644 discloses a CHA-type molecular sieve designated SSZ-62 and having a crystallite size of 0.5 micron or less. SSZ-62 is reported to have a SiO2/Al2O3 molar ratio of 10 or greater. Synthesis is effected in an hydroxide medium in the presence of a N,N,N-trimethyl-1-adamantammonium cation as a structure directing agent.
U.S. Pat. No. 7,148,172 discloses a CHA-type aluminosilicate having a SiO2:Al2O3 molar ratio in excess of 100 (e.g., from 150 to 2000) which is synthesized in the presence of fluoride ions. Structure directing agents employed include N-alkyl-3-quinuclidinol, N,N,N-trialkyl-1-adamantammonium cations and N,N,N-trialkyl-exoaminonorbornane.
U.S. Patent Application Publication No. 2007/0286798 discloses the preparation of CHA-type molecular sieves using various SDAs, including a N,N,N-trimethyl-2-adamantammonium cation.
However, these SDAs are complex and costly, which makes the synthesis of CHA-type molecular sieves using these SDAs complex and costly. This cost can limit the usefulness of CHA-type molecular sieves in commercial processes. Thus, it would be desirable to find a way to reduce or eliminate the use of these costly SDAs in the synthesis of high-silica CHA-type molecular sieves.
It has now been found that CHA-type molecular sieves can be prepared using a N,N-dimethyl-3,5-dimethylpiperidinium cation structure as a partial substitute for these complex and costly SDAs.